Method for measuring properties of flowing fluids, and a metering device and a sensor used for performing this method

ABSTRACT

A method for measuring properties of a flowing fluid composition comprising at least two different components, while the fluid composition is flowing through a duct or channel, which flow meter comprises: at least one sensor ( 2, 3 ) in the shape of a cavity resonator through which at least a portion of the fluid composition passes; at least one electronic circuit ( 4 ) comprising a transmitting means ( 11 ) adapted to transmit an electronic signal ( 9 ) into the flowing fluid via a probe ( 7, 11 ); at least one receiving means adapted to receive a signal which has traveled through the flowing fluid composition; and at least one signal processing unit ( 12 ) adapted to deduce fluid specific signals from the received signals. An oscillator included in the electronic unit ( 4 ) may, in a preferred embodiment, be phase locked to the resonant frequency of the sensor ( 2, 3 ) (in a so-called FSA (feedback self-oscillating amplification) method). The frequency is then counted downhole and the result is transferred to the surface as a digital number for each measurement of the desired properties. The invention also relates to a meter design and to a sensor device designed to fit into the narrow annulus of a subsurface production well.

[0001] The invention relates to a meter and a sensor concept, and alsoto a method for measuring properties of a fluid flow. The metercomprises a sensor, an electronic unit, and a software package, inparticular for the continuous measurement of the composition of a fluidflowing through a duct or channel. In this document in particular theuse as a downhole meter for the measurement of the water fraction of thefluid being produced in an oil well is described. However, the inventionmay also be used for measuring other properties and values.

[0002] The sensor is based on using both the microwave resonanceprinciple for the measurement of oil-continuous fluids (water drops andgas bubbles in oil, i.e. the oil is the continuous phase), and themeasurement of conductivity for water-continuous fluids (oil drops andgas bubbles in water, i.e. the water is the continuous phase). The meteris intended for installation at a production zone inside an oil well.

[0003] The meter can also measure the fluid that is being produced froma specific zone. The measurement result can be used for controlling avalve that controls the production rate from the zone. Such equipment isespecially useful in so-called smart wells, in which several zonesproduce into the same well. The in-flow from one zone is being mixedwith the main flow (that has been produced from other zones and flows ina tube system) at the valve controlling that zone. The composition oughtto be measured inside the well (downhole) between the perforations inthe casing or liner and the valve, while the fluid is flowing in theannulus, before it is being mixed with the main flow. Knowing thecomposition of the fluid being produced is important for the long-termoptimization of the recovery from a zone.

[0004] There are a number of different meters on the market for themeasurement of the water contents of oil. Some meters are based on theuse of radioactive radiation, some are capacitive, and some are based onthe use of microwaves. The radioactive sensors are problematic in manyenvironments because of the health risks with such radiation, and thesafety measures required. In downhole applications this would encountera serious problem, particulary during the installation phase. Inaddition the accuracy is a problem because the radiation is mainlysensitive to differences in density, and the difference in density ofoil and water is small or even zero. The capacitive sensors measure thepermittivity (ref. chapt. 2 in Nyfors E., and P. Vainikainen, IndustrialMicrowave Sensors, Norwood, Mass.: ArtechHouse, 1989) of the fluid atfrequencies that are much lower than those used by microwave sensors.They are therefore very sensitive to all kinds of contamination, as athin layer of e.g. scale or wax has a large influence on the impedanceof such sensors. They also require a relatively complicated mechanicalstructure including a dielectric protecting cover on the inside of thesensor, so that the electrodes do not come in direct contact with thefluid that is measured. Microwave sensors do not have these problems. Amicrowave sensor measures the permittivity of a fluid. Because thepermittivity of water is much higher than that of most other substances,oil included, the permittivity of a fluid containing water is verysensitive to the water content.

[0005] Currently there are no sensors of any kind available on themarket for the measurement of the water content of fluids neither in theannular space between two pipes, nor downhole in an oilwell.

[0006] The conditions that a meter for measuring operations downhole inthe annulus of an oilwell has to face are difficult. These are the highpressure (typically up to 1000 Bar), the high temperature (typically upto 180° C.), the very limited space, the low electrical power available(because of the typically several km long supply cable), and the highrequired reliability because of no possibilities to service the meteronce it has been installed with the completion of the oilwell. Theseconditions require a specially designed microwave sensor for theannulus. The sensor must also be designed so that a measurementprinciple can be used that requires an absolut minimum amount ofelectronics at the downhole location of the sensor because of thelimited space, the limited power, and the requirement on reliability incombination with the high temperature. The measurement method shouldalso be such that a minimum amount of information needs to betransferred to the surface, because of the low data handling capacity ofa several kilometers long combined power/signal cable.

[0007] The resonant frequency of a microwave resonator sensor can bemeasured with basically two different methods (ref. Vainikainen, P.,“Measurement electronics of industrial microwave resonator sensors”,Thesis for the degree of Doctor of Technology, Helsinki University ofTechnology, Radio Laboratory, Report S 194, 1991).

[0008] The first involves measuring the frequency response of theresonator by performing a frequency sweep, with e.g. a VCO (voltagecontrolled oscillator). The resonant frequency is then usually derivedby performing a curve fit. If this method would be used in a downholeapplication, either the whole frequency response, typically involvinghundreds of measurement points, would have to be transferred to thesurface for one measurement of the water content, or there would have tobe a data processing capability in the downhole electronics.Transferring the frequency response to the surface is slow, andprocessing the data downhole would make the electronic unit much morecomplex and unreliable.

[0009] The second is the so-called FSA method (feedback self-oscillatingamplifier), which is based on locking an oscillator to the resonantfrequency of the sensor. The FSA method is fast and simple. Thefrequency only needs to be counted downhole and transferred to thesurface as a single number for each measurement of the water content.The FSA method requires a pure frequency response in the sense thatthere should be no other resonance peaks near the one used, so that itcan be assured that the electronics always locks to the right resonancepeak.

[0010] This invented sensor is a microwave cavity resonator of a newdesign, in particular adapted for downhole use, e.g. in an oil/gas well.The advantages are that it is suitable for permanent installation in anannulus, the frequency response that makes it suitable for measurementwith the FSA method, a simple structure, and probes that can also beused for measuring the conductivity of the fluid for determining thewater content, when the fluid is water-continuous. With these features aminimum amount of electronics is required downhole, which allows forobtaining a good reliability also at the high temperatures encounteredin the downhole environment. A minimum amount of data transmission isneeded, which enables for fast measurements. All software for performingthe necessary calculations of the water content can be located at thesurface.

[0011] These advantages can be obtained by using the techniques,features and methods according to the claims stated below.

[0012] A more detailed description of some embodiments of the inventionis given below and a preferred embodiment is in particular shown. Theapplication as a permanent downhole meter for measuring the watercontent of the in-flow from a single zone in an oilwell is used as anexample. The following figures are used in the description:

[0013]FIG. 1 The cross section of a semisectorial waveguide.

[0014]FIG. 2 The design of the downhole semisectorial cavity resonatorsensor, shown by an longitudinal view and one cross section.

[0015]FIG. 3 The cross section of an oil well with two sensors of theabove type mounted in the annulus in the well.

[0016]FIG. 4 The electric field intensity in the sensor of the TE modeswith n & 1≦3, m=1 projected on a cylindrical folded out surface. Thelocation of the probes are marked with crosses.

[0017]FIG. 5 The design of the probes seen in cross section.

[0018]FIG. 6 The frequency response of the sensor as measured with anetwork analyzer. The salinity of the water is 3%. The water contentvaries stepwise from 0% to 25%.

[0019]FIG. 7 The principle of the FSA method of measuring the resonantfrequency.

[0020] In the figures the same reference numerals are used for identicalor similar components and elements when found applicable. It should alsobe noted that the figures are drawings primarily depicting theprinciples and accordingly some practical details may be omitted even ifrequired to implement the invention. Eventually it should be mentionedthat the scale is not the same on each drawing, not even in differentparts or different directions of one single drawing.

[0021] In FIG. 1 the main shape of a so-called semisectorial sensor 2 isshown as a cross section. The shape of a sensor 2 may thus be defined bythe angle φ, the inner radius b and the outer radius a. The surroundingwall 21 of the sensor 2 is made up of a rigid and electric conductingmaterial having a hollow design so that the fluid may pass freelythrough the “curved tube”, perpendicularly to the paper plane. Thenumber of semisectors which may be arranged within an annular space,e.g. between a production tube and a casing in an oil or gas well, willdepend on the angle φ.

[0022] The length of the sensor 2 is shown on FIG. 2, which on left handshows a view taken along the axis of the sensor and on right hand showsan end view of the sensor. Here it is assumed that end grids 18, alsoconducting, are arranged at both ends of the sensor 2. As it is seen,the medium portion of the sensor includes only one single chamber, whilethe end grids only are introduced at the end portions to screen off thecavity of the resonator from the surroundings.

[0023] In FIG. 3, there is shown as a preferred embodiment that two suchsensors 2,3 are aranged as an integral unit within the annulus of aproduction tube, preferably downhole in an oil/gas well. Then also highfrequency equipment may be included in the downhole unit, while allother equipment such as the electronic circuit 4 and possible computingor signal treating units (12) may be arranged at the surface.

[0024]FIG. 4 shows the field within the sensor for different wave modi.The x marks visible in the diagrams represent the location of the probesused in an PSA operation, as explained below.

[0025] In FIG. 5 an embodiment of a probe 7,11 is shown in more detail.Here a coaxial probe construction is assumed, but other conventionalprobe designs may be used as well.

[0026] On FIG. 6 the frequency response is shown as explained later inthe specification, and in FIG. 7 the FSA method is shown in principle,as explained furhter down.

[0027] The invention is recognized by that when the permittivity of afluid is measured for determining the composition, a microwave cavityresonator is used. The resonator is a semisectorial cavity resonatorwith end structures consisting of radial-axial plates or end grids 18,which allows the flow to pass through the sensor 2 unhindered. Theprobes are located so that coupling to the resonance modes in thevicinity of the used mode is eliminated, making the sensor well suitedfor the FSA method of measuring the resonant frequency. The same probesthat are used for coupling the microwave signal to the sensor can alsobe used for measuring the conductivity of the fluid, in case of e.g. awater-continuous fluid consisting of oil, water, and gas. The softwarefor calculating the composition may be located at a large distance, e.g.at the surface in the case of the application of measuring downhole inan oilwell, because of the small amount of data transfer needed. Infront (upstreams) of the sensor there may be a mixer 17, of anyconventional type, that assures that the fluid is well mixed in case thefluid has segregated while flowing in the annulus.

[0028] The pressure drop over this mixer, or some other part in theannulus creating drag, may also be measured and the flow speed derivedfrom an empirically calibrated model. From the flow speed and thecomposition measurement, production rates of e.g. water and hydrocarbonscan be calculated.

[0029] The calculation methods will be explained below:

[0030] When two material components (A and B), (liquid, gas, or solidparticles), with different permittivity (ε_(A) and ε_(B)) are mixed, themixture has a permittivity ε_(m) that is dependent on the mixing ratio φof the two components (ref. chapt. 2 in Nyfors E., and P. Vainikainen,Industrial Microwave Sensors, Norwood, Mass.: ArtechHouse, 1989). Themixing ratio is usually expressed as the total volume of one of thecomponents relative to the volume of the mixture, e.g. $\begin{matrix}{\varphi_{A} = \frac{V_{A}}{V_{A} + V_{B}}} & (1)\end{matrix}$

[0031] where V_(A) is the volume of component A and V_(B) is the volumeof component B in a sample of volume V_(m)=V_(A)+V_(B) of the mixture.If e.g. A is water and B is oil, φ_(A) is the water content of themixture. In the case of the fluid produced in an oil well, B may in turnbe a known mixture of oil and gas, and will therefore be generallycalled the hydrocarbon component. The way ε_(m) depends on φ depends onhow the components mix with each other and is therefore specific forthese components. As a model for this dependence a known model (ref.chapt. 2, 4 in Nyfors E., and P. Vainikainen, Industrial MicrowaveSensors, Norwood, Mass.: ArtechHouse, 1989), may be used, or anempirical calibrated model. By using this model, φ can then be derivedfrom a measured value of ε_(m).

[0032] For the measurement of ε_(m), a microwave resonator can be usedas a sensor. Such a sensor has a resonant frequency that is dependent onthe permittivity of the medium with which it is filled. If f₀ is theresonant frequency of the sensor, when it is empty, and f_(m), when itis filled with the mixture, the permittivity is, as stated in Nyfors E.,and P. Vainikainen, Industrial Microwave Sensors, Norwood, Mass.:ArtechHouse, 1989. $\begin{matrix}{ɛ_{m} = \left( \frac{f_{o}}{f_{m}} \right)^{2}} & (2)\end{matrix}$

[0033] It is previously known that microwave resonators for themeasurement of fluids can be made of cylindrical pipes with relativelyopen end grid structures that allow the fluid to flow through thesensor, but prevents the microwaves from escaping. Ref. e.g. U.S. Pat.No. 5,103,181 (Gaisford et al). The present invention comprises a newtype of microwave cavity resonator sensor, shaped to fit into theannulus and with end grid structures. The cross section of the sensor isthe part of a sector that is limited between two concentric circles.Such a shape will here be called a semisector (FIG. 1).

[0034] The new sensor is thus a semisectorial cavity resonator sensor.It should be emphasized that there may be used one, two or more than twosuch semisectorial elements within the annulus. Accordingly “semi” doesnot refer to the half of the cirumferial, but rather to any portion ofthe perimeter. The space not filled up with a waveguide may be used forcables, connectors, etc.

[0035] In hollow waveguides, i.e. electrically conducting pipes,microwaves can propagate in various wave modes called TE or TM wavemodes (ref. chapt. 3 in Collin, R. E., Foundations for MicrowaveEngineering, New York: McGraw-Hill, 1966) having specific cut-offfrequencies. Power can propagate on a specific mode only at frequenciesabove the cut-off frequency of that mode. The modes in semisectorialwaveguides can be solved following the same procedure as for cylindricalwaveguides. The modes are then called TE_(vn) or TM_(vm), and theircut-off frequencies are given by $\begin{matrix}{{f_{c \cdot {vm}} = \frac{c \cdot P_{vm}}{2\pi \quad a}},\quad \left( {TM}_{vm} \right)} & (3) \\{{f_{c \cdot {vm}} = \frac{c \cdot P_{vm}^{\prime}}{2\quad \pi \quad a}},\quad \left( {TE}_{vm} \right)} & (4)\end{matrix}$

[0036] where c is the speed of light in vacuum (3×10⁸ m/s), and a is thelarger radius of the semisector (FIG. 1). The index νis given by$\begin{matrix}{v = \frac{n\quad \pi}{\varphi_{0}}} & (5)\end{matrix}$

[0037] where φ₀ is the sector angle (FIG. 1), and n is an integer (n=0,1, 2, . . . (TE), and n=1, 2, 3, . . . (TM)). P_(vn)=k_(c)a is the m:thsolution to the equation $\begin{matrix}{{{\frac{J_{v}\left( {k_{c}a} \right)}{J_{v}\left( {k_{c}b} \right)} \cdot \frac{Y_{v}\left( {k_{c}b} \right)}{Y_{v}\left( {k_{c}a} \right)}} - 1} = 0} & (6)\end{matrix}$

[0038] where J_(ν) and Y_(ν) are Bessel functions of the first andsecond kind and order ν, b is the smaller radius (FIG. 1) and$\begin{matrix}{k_{c} = \frac{2\pi \quad f_{c}}{c}} & (7)\end{matrix}$

[0039] P′_(m)=k_(c)a is the m:th solution to the equation$\begin{matrix}{{{\frac{J_{v}^{\prime}\left( {k_{c}a} \right)}{J_{v}^{\prime}\left( {k_{c}b} \right)} \cdot \frac{Y_{v}^{\prime}\left( {k_{c}b} \right)}{Y_{v}^{\prime}\left( {k_{c}a} \right)}} - 1} = 0} & (8)\end{matrix}$

[0040] where the apostrophe denotes the derivative with respect to theargument of the function. In the general case Eqs. (6) and (8) can onlybe solved numerically.

[0041] A microwave resonance mode in a semisectorial cavity resonator isbased on a TE_(vm) or TM_(vm) waveguide mode. The resonator is a lengthL of the semisectorial waveguide bounded by end structures that providean open or short circuit to the wave mode, so that the waves arereflected back and forth generating a standing wave pattern in thebounded waveguide section. The wave mode gets a third index l associatedwith the length L of the resonator. The resonant frequency of thevarious modes is then (ref. Nyfors E., and P. Vainikainen, IndustrialMicrowave Sensors, Norwood, Mass.: ArtechHouse, 1989, p 150).$\begin{matrix}{f_{r,{vm1}} = {\frac{c}{2}\left\lbrack {\left( \frac{x_{vm}}{\pi \quad a} \right)^{2} + \left( \frac{1}{L} \right)^{2}} \right\rbrack}^{\frac{1}{2}}} & (9)\end{matrix}$

[0042] where x_(vm) denotes P_(vm) or P′_(vm). The invention is aresonator with short-circuiting ends and can therefore support TM_(vml)modes with index l=0, 1, 2, . . . and TE_(vml) modes with l=1, 2, 3, . .. .

[0043] In the invention a microwave semisectorial cavity resonatorsensor has been designed for measuring the water content of the fluidproduced by an oilwell, while the fluid is flowing in the annulus. Anexample of the resonator is shown in FIG. 2, and FIG. 3 shows a crosssection of the oil well with two such sensors mounted on the outside ofthe tubing. The sensor has a small clearing to the casing, which isnecessary when the tubing (incl. sensors et.c.) is slided in placeduring completion of the oilwell. Two sensors can be used as shown inFIG. 3 to improve the reliability by providing redundancy, or to improvethe sampling in case of segregation. Two sections of the circumferenceof the tubing have been left free in FIG. 3 to allow space for cablesetc. bound for other production zones deeper in the well to pass.

[0044] The embodiment shown in FIG. 2 may be used for different wellsizes, however, the sensor shown was designed for a well with a 4″tubing in a 7″ liner (a 7″ casing is called a liner). Table 1 shows theresonant frequency of the 10 modes with the lowest resonant frequency,and FIG. 4 shows a qualitative picture of the electric field intensityof the TE modes with n & l≦3, m=1 projected on a cylindrical surface.The mode with the lowest resonant frequency (TE_(vll), n=1) was chosenfor the measurement purposes, because it is the most practical choice,especially when the FSA method is used. TABLE 1 The resonant frequencyof the 10 modes with the lowest resonant frequency in a semisectorialmicrowave cavity resonator sensor with the dimensions: a = 72 mm, b = 62mm, φ₀ = 128.3°, and L = 225 mm. ν was given by (5), P′_(ν1) by (8), andƒ_(r) by (9) . Note that the P_(νm) values calculated from (6) show thatall TM modes in the resonator have higher resonant frequencies thanthose shown in the table. Mode n ν P′_(ν1) f_(r) TE_(n11) 1 1.403 1.5091.202 TE_(n12) 1 1.403 1.509 1.667 TE_(n11) 2 2.806 3.018 2.110 TE_(n13)1 1.403 1.509 2.236 TE_(n12) 2 2.806 3.018 2.405 TE_(n13) 2 2.806 3.0182.829 TE_(n14) 1 1.403 1.509 2.848 TE_(n11) 3 4.209 4.527 3.075 TE_(n12)3 4.209 4.527 3.285 TE_(n14) 2 2.806 3.018 3.334

[0045] When measuring the resonant frequency of a resonator sensor withthe FSA method, two coupling probes are needed. Because the used modehas a purely radial electric field, coupling probes of the electric type(ref. Nyfors E., and P. Vainikainen, Industrial Microwave Sensors,Norwood, Mass.: ArtechHouse, 1989) located radially is the naturalchoice. The basic design used in the invention is shown in FIG. 5. Inthe realized design the probes are mounted on the concave cylindricalsurface in the locations indicated by the crosses in FIG. 4. One hasequal distance to the broad ends and is displaced ⅓ of the distance fromthe centre towards one short end, and the other one has equal distanceto the short ends and is displaced ⅓ of the distance from the centretowards one broad end. In these positions at least one of the probes isalways in a null of a mode with at least one even numbered index (n or1), or at least one index equal to 3, so avoiding coupling to thesemodes. This is also clearly seen in FIG. 4 from the fact that maximumone of the crosses is visible, except for the used mode. With the probesmounted in the indicated positions, a frequency response is achievedthat is well suited for the FSA method. FIG. 6 shows the frequencyresponse of the sensor as measured with a network analyzer, when thesensor is filled with various mixtures of oil and water of various watercontent. It can be concluded that there are no confusing resonance peaksin the vicinity of the used peak. The sensor is therefore well suitedfor measurement with the FSA method.

[0046] The end grid structure of the sensor consisting of radial/axialplates is shown in FIGS. 2 and 3. The plates basically divide thesemisectorial waveguide into 5 smaller semisectorial waveguides. Becauseof the smaller sector angle they will have a higher cut-off frequency.In the shown design the cut-off frequency of the grids is 5 GHz in air.Because this is higher than the used resonant frequency of 1.2 GHz(Table 1), the microwaves cannot escape through the end grids. Becausethe below-cut-off attenuation is finite, the end grids need to have afinite length. The shown 50 mm has been found to be enough. When thesensor is filled with the measured fluid, both the resonant frequencyand the cut-off frequency of the grids will change according to equation(2), but the ratio will be constant. Therefore the grids will be tightunder all conditions. The grids create almost no blockage to the flow,which makes this sensor ideal for measuring a flowing fluid.

[0047] The FSA method was described in detail in Norwegian patentapplication No. 98.2538, and will be described only briefly here. Theprinciple of the FSA method is shown in FIG. 7. The output of anamplifier 23 is coupled to one probe 7 of a resonator sensor. The signalis received with the other probe 11 and fed back to the input of theamplifier 23. When the insertion loss in the sensor is lower than thegain in the amplifier, there is net gain in the circuit, which leads tooscillation. If the amplifer has a gain that is falling with frequency,the used resonance is the lowest one, and there are no other resonancesnear the used one, i.e. oscillation is possible only at the rightresonance peak. In addition to the gain condition there is, however,also a phase condition for oscillation: The total phase changeexperienced by the signal during one revolution in the circuit must be

Δφ=n·360°  (10)

[0048] where n is an integer. This means that the circuit generally doesnot oscillate exactly on the resonant frequency, but on the nearestfrequency where the phase condition is fulfilled. When the compositionchanges so that the resonant frequency changes, the oscillation jumpsfrom one frequency (n) to the next (n±1) in steps. Because the phasechange is dominated by the phase change in the cables, the size of thestep depends primarily on the length of the cables: $\begin{matrix}{{\Delta \quad f} = {{\frac{\left( {n + 1} \right)c}{d\sqrt{ɛ_{c}}} - \frac{n \cdot c}{d\sqrt{ɛ_{c}}}} = \frac{c}{d\sqrt{ɛ_{c}}}}} & (11)\end{matrix}$

[0049] where d is the total length of the cables and ε_(c) is thepermittivity of the insulating material in the cables. Because of thephase change in both the sensor and the electronics the real step isslightly smaller. In practice the discrete nature of the FSA methodmeans that the resolution of the resonant frequency measurement islimited primarily by the length of the cables. If d=10 m and ε_(c)=9,

Δf=10 MHz. Because the sensitivity of the sensor is roughly 10 MHz/%(water), the resolution is roughly 1% (water), which is sufficient forthe downhole application.

[0050] The sensor has been tested in a test loop filled with crude oiland water with various salinity (S=0 . . . 15%). The resonant frequencywas measured both with a network analyzer and an FSA electronics thatwas built to fit into a housing with an inner diameter of 19 mm, and tobe used downhole. The results were very good. The difference in theresonant frequency measured with the network analyzer and that measuredwith the FSA downhole electronics was in accordance with equation 11.The sensor worked well through the whole oil-continuous range (up to76%(water) in this experiment) The difference between one curve and thenext in this diagram corresponds to approx. 5%. In the water-continuousrange the conductivity of the fluid was measured by measuring the loadresistance of one of the probes. The results show that the measured loadresistance can be calibrated against the water content, when theconductivity (salinity and temperature) of the water is known. Therebythe same sensor can be used also with water-continuous fluids (usuallyat the end of the life of an oilwell) by adding a simple electroniccircuit that measures the load resistance of one of the probes.

[0051] A sensor has been described that is based on a microwavesemisectorial cavity resonator. The end grid structure of the sensorcreates no blockage to a flowing fluid, and the sensor fits into theannulus of an oilwell. Because of the location of the probes the sensoris well suited for measurement using FSA electronics, which has beenbuilt to fit into a housing that also fits into the annulus. Because ofthe design of the probes they can also be used for measuring thecomposition in water-continuous fluids based on measuring the loadresistance. The sensor is therefore suitable for measuring thecomposition of the inflow from a production zone in a smart well.

[0052] The invention may be modified in many manners. More than onesectorial cylinder may be integrated in each curved unit, or with otherwords each sensor 2,3 may comprise not only one single but two orseveral parallel cavity resonators, each with its own probes. Suchparallel resonators 2,3 may be separated by common partition walls asopposite sides of one single wall may be used in each adjacentresonator. Each resonator must be provided with separate probes.However, the probes must not necessarily have the design as shown inFIG. 5. Instead the probes may have any suitable design determined byskilled persons according to given requirements.

[0053] The complete meter may comprise one single downhole unit in whichone single or more than one sensor are arranged. This downhole unit maythen also include necessary cabeling and high frequency somponents,converters and interfaces, so that the unit may be deployed in onesingle operation.

[0054] If many small dimension sensors are arranged along the perimeter,a more or less geometric approach is feasible as some of the sensorsthen will have a flow of nearly pure water, while other of the sensorswill have a flow of nearly pure oil, due to segregation. The principlethen will be to count the number of sensors with mainly oil and thenumber of sensors containing mainly water. The accuracy of the resultwill then increase with the number of sensors. A mixer at the input thenmay be omitted, and the degree of segregation may also be determined.

[0055] Still another approach is to use only one probe in each sensorand then measure the reflected energy. Then an inverted pike (downwarddirected response) will be registered on the resonance diagram. Themethod used with only one probe then will be the frequency responsemethod and not the FSA method, which in other cases is deemed to be themost advantageous, however, not the only feasible method.

1. A method for measuring properties of flowing fluids comprising atleast two different components, while the fluid composition is flowingthrough a duct or channel, which flow meter comprises at least onesensor (2,3) in the shape of a cavity resonator through which at least aportion of the fluid composition passes, at least one electronic circuit(4) comprising a transmitting means (5) adapted to transmit anelectronic signal (6) into the flowing fluid via a probe (7), at leastone receiving means (7,11) adapted to receive a signal having traveledthrough the flowing fluid composition, and at least one signalprocessing unit (12) adapted to deduce fluid specific signals from thereceived signals, characterized in that an oscillator included in theelectronic unit (4) is phase locked to the resonant frequency of thesensor (2,3)(in a so-called FSA-(feedback self-oscillatingamplification) method, that the frequency is determined downhole and theresult is transferred to the surface as a number representing eachmeasurement of the desired property(ies).
 2. A method according to claim1, characterized in that the pressure drop over the meter (1) or some ofits components is measured, that the flow speed is derived from thismeasurement using an empirically calibrated model, and that theseresults are combined with the properties found by the micro wavemeasurement to give separate production rates of water and hydrocarbons.3. A flow meter for continuous measuring properties of at least onefraction included in a flowing fluid composition comprising at least twodifferent components flowing through a duct or channel, which meter (1)comprises at least one sensor (2,3) designed as a wave guide throughwhich the fluid composition (10) flows, at least one electronic unit (4)comprising a transmitting means (5) adapted to transmit an electricsignal (6) into the fluid flowing within the sensor fluid (10) via aprobe (7,11) arranged therein, at least one receiving means (8) adaptedto receive a signal (9) by means of a probe (7,11) arranged in thesensor (2,3), and at least one signal processing unit (12) adapted todeduce fluid specific property values (13), from the received signals(9), characterized in that the sensor(s) (2,3) is(are) designed to fitinto an annular space (14) between one external tube (15) and aninternal core (16) arranged therein, so that at least a portion of thefluid composition (10) flows through the sensor (2,3).
 4. A flow meteras claimed in claim 3, characterized in that the sensor(s) (2,3)comprise(s) at least one conducting, thin-walled, hollow cylindricsector (2,3) having dimensions allowing one or more such sensorsarranged adjacent to each other or separated from each other within theannular space (14) between the production tube (16) in a subsurfacehydrocarbon producing well and the outer casing (15); so that a portionor all of the fluid (10) flowing through said annulus (14) passesthrough at least one of these hollow sylindric sectors (2,3).
 5. A flowmeter according to claim 3 or 4, characterized in that a mixer (17) isarranged upstream of each sector shaped sensor.
 6. A flow meteraccording to one of the claims 3-5, characterized in that each sensor(2,3) comprises more than one semi sectorial cavity resonator.
 7. A flowmeter according to one of the claims 3-6, characterized in that eachsensor (2,3) is provided with an end grid (18) at each of its ends (19)allowing the flow to pass unhindered, but stopping the micro waves.
 8. Aflow meter as stated in one of the claims 3-7, characterized in thateach sensor is provided with at least one internal probe (20) adapted totransmit and/or receive micro wave energy into/from each cavityresonator (2,3), said probe(s) (20) being located at a null-point for aspecific oscillating mode, to avoid coupling to specific predeterminedmodes.
 9. A sensor adapted for downhole use, in particular in asubsurface production well, characterized in that the sensor(s) (2,3)is(are) designed to fit into an annular space (14) between one externaltube (15) and an internal core (16) arranged therein, so that at least aportion of the fluid composition (10) flows through the sensor (2,3).10. A sensor as claimed in claim 9, characterized in that the sensor(s)(2,3) comprise(s) at least one conducting, thin-walled, hollow cylindricsector (2,3) having dimensions allowing one or more such sensorsarranged adjacent to each other or separated from each other within theannular space (14) between the production tube (16) in a subsurfacehydrocarbon producing well and the outer casing (15); so that a portionor all of the fluid (10) flowing through said annulus (14) passesthrough at least one of these hollow sylindric sectors (2,3).